A forward converter is a DC-to-DC converter that uses a transformer to increase or decrease the output voltage and to provide galvanic isolation for the load. Although its circuit topology is similar to that of the well-known flyback converter, a forward converter operates in a different way. A flyback converter stores energy as a magnetic field in a transformer during the time the main switch is on. When the switch is on, current is flowing through the primary winding of the transformer. When the main switch turns off, the magnetic field decreases and energy is transferred from the transformer to the output of the flyback converter as electric current. In contrast, a forward converter ideally stores no energy in its transformer during the time the main switch is on. Rather, when the main switch of a forward converter is turned on energy passes through the transformer directly to the output of the forward converter by transformer action. Ideally energy flows through transformer but is not stored in the transformer. The main switch is then turned off, but current continues flowing to the output capacitor and load due to an inductance disposed in the output current path. The inductance is disposed between the transformer secondary winding and the output capacitor. As the energy stored in the inductance is removed over time, the inductor current decreases and the rate of energy transfer to the output decreases. In the next switching cycle, when the main switch is turned on the inductor current starts increasing again and the rate of energy transfer to the output increases again. The output voltage across the output capacitor is regulated by controlling the on time of the main switch.
The transformer of the forward converter is not, however, ideal. An ideal transformer has infinite magnetizing inductance but a practical transformer has a limited magnetizing inductance. As a result, in the forward converter, when the main switch is turned off some magnetization energy remains stored in the transformer. If the remaining magnetization energy is not removed from the transformer before the next switching cycle is started, then the magnetization energy remaining in the transformer core will accumulate from cycle to cycle until the core saturates. The transformer will not work properly with a saturated core. A current path is therefore provided to remove trapped energy from the transformer before the main switch is closed again at the beginning of the next switching cycle. A tertiary winding is often provided for this purpose. A diode is coupled between the tertiary winding and the input capacitor. If during demagnetization the voltage across the tertiary winding exceeds the input DC voltage on the input capacitor, then a current flows back from the tertiary winding, through the diode, and back into the input capacitor. This current transfers the magnetization energy from the transformer back into the input capacitor. As the magnetization energy in the core is removed, the current flow through the diode decays. When all the magnetization energy in the core has been removed, the core is said to have been “reset”. To ensure that the core is in fact reset and that residual energy does not remain, an amount of “idle” time is generally provided between the time when the core is believed to have been reset and the time when the next switching cycle is initiated. After the idle time has expired, the main switch is turned on again to start the next cycle. One switching cycle therefore involves: 1) a time Ton when the main switch is on, 2) a time Tdemag when the main switch is off and the core is being demagnetized, and 3) and an idle time Tidle after core demagnetization while the main switch is still off.
FIG. 1 (Prior Art) is a circuit diagram of an AC-to-DC converter circuit 1 that includes a forward converter. Circuit 1 includes a full-bridge rectifier 2, a power factor correction circuit 3, an input capacitor 4, a main switch 5, a switch driver circuit 6, a transformer 7, a demagnetization path diode D1 8, a rectification diode D2 9, a free-wheeling diode 10, an output inductor L1 11, and an output capacitor 12. The AC-to-DC converter circuit 1 receives a 110 volt RMS AC input supply voltage VS 13, and supplies a load 14 with a 2.5 volt DC output voltage VO 15. Transformer 7 includes a primary winding 16 having N1 turns, a secondary winding 17 having N2 turns, and a tertiary winding 18 having N3 turns. The switch driver circuit 6 has a startup power connection (not shown) and a power connection to the output voltage of the secondary side (not shown). Details of the switch driver circuit 6 and output regulation circuitry are not shown.
FIG. 2 (Prior Art) is a simplified diagram that illustrates the relationship of the windings 16, 17 and 18 of the transformer 7. A practical realization of transformer 7 may take on a different form. The diagram of FIG. 2 is presented here for instructional purposes.
FIG. 3 (Prior Art) is a diagram that illustrates the waveform of the sinusoidal input supply voltage VS 13. The input supply voltage VS is a 110 volt RMS, 60 Hz signal having a peak voltage of about 156 volts. The DC output voltage VO 15 in this example is 2.5 volts DC signal. Waveform VI 19 represents the voltage across the input capacitor 4. Each half period of the incoming sinusoidal voltage VS 13 has a duration of about 8.3 milliseconds. The main switch 5, however, has a switching cycle of 10 microseconds. There are many switching cycles of the main switch in each 8.3 millisecond half period of the incoming AC supply voltage.
In a switching cycle, when the main switch is turned on, the rough DC voltage VI is applied to the primary winding and simultaneously a scaled voltage appears across the transformer secondary winding. The input voltage VI to output voltage VO ratio is set by the primary-to-secondary turns ratio of the transformer. Diode D2 is forward biased. A charging current 20 flows through inductor L1 and charges the output capacitor 12. This current 20 increases over time while the main switch is closed. When the main switch is then turned off, the primary winding current and the secondary winding current both fall to zero. Current 20 through the inductor L1, however, continues flowing. Current 20 does not flow through rectifier diode D2, however, but rather flows from ground node 21, up through free-wheeling diode D3, through the inductor L1, and to output node 22 and the output capacitor 12 and load 14. The required emf to maintain this current 20 flowing when main switch is off comes from the inductor L1. Current 20 decreases over time when the switch is off. Despite the increasing and decreasing inductor current as the main switch turns on and off, the large output capacitor 12 maintains a relatively constant DC output voltage VO 15 across the load 14. The switching cycle then repeats.
The rectification diodes D2 and D3 suffer conduction losses. The forward conduction loss of a diode is equal to the product of the forward voltage drop across the diode and the forward conduction current. Such forward conduction losses contribute to the overall power loss of the forward converter circuit. By replacing each of the rectifier diodes D2 and D3 with a field effect transistor that operates as a synchronous rectifier, the equivalent forward voltage drops across the rectifiers can be lowered. Due to the lowered forward voltage drop, the conduction loss of the rectifier is reduced and efficiency of the overall forward converter circuit is increased.
FIG. 4 (Prior Art) is a diagram of a common forward converter circuit 23. The circuit 23 of FIG. 4 is similar to the circuit of FIG. 1, except that the diodes D2 and D3 of the simple circuit of FIG. 1 are replaced with field effect transistors 24 and 25, respectively. As shown, in field effect transistor has an inherent body diode. Body diode 26 is the body diode of transistor 24. Body diode 27 is the body diode of transistor 25. The field effect transistors 24 and 25 that are controlled to take the place of the rectifying diodes D2 and D3 in the circuit of FIG. 1, are referred to as synchronous rectifiers. At times when the diodes D2 and D3 in the circuit of FIG. 1 would be on and conducting, the field effect transistors 24 and 25 of the circuit of FIG. 4 must be supplied with appropriate gate signals so that these transistors are on. At other times, the gates of the transistors 24 and 25 should be driven so that the transistors are off. In the conventional circuit of FIG. 4, the voltage across the secondary winding 17 is used to drive the gates of the synchronous rectifiers Q1 and Q2 so that the transistors are on and conductive at the appropriate times, and are off and non-conductive at the appropriate times. Because there is no extra active drive circuit involved in driving the gates of the field effect transistors, the field effect transistors are said to be “self-driven”. The forward converter circuit is said to be a forward converter with self-driven synchronous rectifiers.
FIG. 5 (Prior Art) is a waveform diagram that illustrates signals and voltages in the forward converter circuit of FIG. 4. The upper waveform labeled VGS,Q1 is the gate drive signal that is supplied onto the gate of the main switch Q1 5. The second waveform labeled VDS,Q1 represents the drain-to-source voltage across the main switch Q1 5. The third waveform labeled VR represents the voltage VR 28 indicated in FIG. 4. The next waveform labeled VGS represents the gate-to-source voltages supplied onto the gates of transistors Q2 24 and Q3 25, respectively. The next waveform labeled IL1 represents the current 20 flowing through the inductor L1 in the circuit of FIG. 4. The bottom waveform labeled VO represents the DC output voltage VO 15.
Unfortunately, as is known in the art, synchronous rectifiers also have conduction losses. There are several different reasons for the conduction losses. Many techniques and circuits are known in the art for reducing conduction losses in synchronous rectifiers. These techniques are sometimes classified to fall into one of three classes: a self-driven synchronous rectifier class, a hybrid self-driven synchronous rectifier class, and a control-driven synchronous rectifier class. The control-driven synchronous rectifier class of circuits, for example, includes so-called predictive gate drive circuits and so-called adaptive gate drive circuits. Despite the existence of many circuits and techniques for reducing conduction losses in the synchronous rectifiers of forward converters, each circuit or technique suffers from one or more drawbacks such as expense, complexity, relative ineffectiveness, and possible catastrophic failure modes.